Understanding and explaining spatio-temporal dynamics of ecological communities and their interaction with the environment is a central goal in ecology, but at the same time a very challenging task. Functional traits are a promising concept to achieve a better mechanistic understanding of variations in community structure. Traits are characteristics of organisms that directly or indirectly influence fitness and vary largely between different species. As traits functionally link communities with their bio-geochemical environment, they are well suited for explaining how community structure changes in response to changing environmental conditions, and how in turn communities shape their surrounding environment. Despite the potential of trait-based approaches to mechanistically explain the relationship between communities and their environment, they have rarely been applied in freshwater ecology. Therefore, this thesis explores spatio-temporal dynamics in functional traits of freshwater phytoplankton communities and their interaction with the environment, using the Rappbode Reservoir in Germany as a case study.
Phytoplankton community data from a seasonally resolved dataset with 50 years of observation from the Rappbode Reservoir were used to translate taxonomic composition into ecologically meaningful functional trait values. The studied functional traits showed consistent, reoccurring seasonal developments that clearly mirrored environmental pressures over the year. From late autumn to spring nutrients and turbulence were high. Hence small celled, fast growing species that are able to rapidly incorporate existing nutrients and tolerate poor light conditions dominated (dominant traits: maximum growth rate, light affinity). In contrast, when turbulence and nutrients were low in summer, large cell size and more complex mechanisms to efficiently exploit mineral nutrients or acquire previously unexploited nutrient pools were key ecological strategies (dominant traits: phosphate affinity, mixotrophy, motility, nitrogen fixation). The similarity in observed trait patterns over several years indicates that despite the diversity and complexity of phytoplankton species dynamics, the seasonal succession is a highly ordered, predictable process, driven by trade-offs between different ecological strategies.
During the 50 years of observation, nutrient concentrations in the Rappbode Reservoir decreased strongly (oligotrophication). A comparison between nutrient-rich and nutrient-deficient years revealed that the general intra-annual succession patterns of phytoplankton functional traits remained the same. However, the intra-annual succession patterns were more pronounced in nutrient-rich years. While functional community composition in summer changed little after oligotrophication, spring communities were largely affected by nutrient reductions and functional traits in spring became more similar to the ones in summer. Based on the quantitative analysis of functional traits, the thesis could generalize the existing patterns to provide a quantitative, functional template for seasonal succession patterns in lake ecosystems under nutrient-rich and nutrient-deficient conditions.
Over the entire observation period, phytoplankton biomass did not respond to reduced phosphorus concentrations. This unexpected resistance of algal biomass against reductions in the limiting resource phosphorus was caused by changes in internal processes and functional traits of the phytoplankton community, allowing them to adapt to lower nutrient levels without a loss in total biomass. The main casual mechanism for the decoupling of inorganic nutrients from phytoplankton biomasses was an increase in phagotrophic mixotrophs and their ability to make bacterial phosphorus available for the entire phytoplankton community. Further, in nutrient-deficient years biomass losses in spring were compensated by high biomasses in summer. Higher summer biomasses compared to spring biomasses in nutrient-deficient years were probably also linked to a decrease in diatoms in spring. Diatoms are characterized by high sinking velocities. This trait in combination with phosphorus uptake by diatom cells leads to high downward nutrient exports from the pelagic zone. The decrease of diatoms in nutrient-poor years was related to less phosphorus losses over the season, leaving more nutrients for summer phytoplankton. The effect of species with high sinking velocities on seasonal phosphorus processing illustrates the importance of functional trait composition on biogeochemical cycling.
A one-year monitoring campaign with a high temporal and depth resolution at Rappbode Reservoir allowed to investigate the impact of vertical phytoplankton trait distributions on the geochemical environment. During summer, a phytoplankton mass development occurred at the depth of the metalimnion, mainly represented by the cyanobacterium Planktothrix rubescens. The positive net growth at larger depth of this metalimnetic species is enabled through the possession of specific traits, e.g. buoyancy regulation and the ability for efficient light harvesting. The data indicated that the occurrence of metalimnetic species was connected to the formation of a metalimnetic oxygen minimum. The oxygen minimum occurred after the metalimnetic algae peak disappeared from the thermocline. Metalimnetic phytoplankton induced oxygen depletion probably through one of the following processes: Oxygen consumption by bacteria related to the degradation of dead organic material from metalimnetic algae or internal respiration of metalimnetic algae towards the end of the growing season leading to net oxygen losses due to the continuation of algal respiration. This constitutes a previously undiscovered mechanism for the development of metalimnetic oxygen minima.
In conclusion, the thesis illustrates how trait-based approaches enhance ecological understanding of phytoplankton community dynamics. The trait-based approaches shed light on how phytoplankton communities respond to environmental gradients and how communities can affect their geochemical environment. The thesis also shows that functional traits can be used to reduce the complexity of communities through converting species information into ecologically meaningful functions and that they allow to link changes in community composition to corresponding features in the biogeochemical environment. The quantitative, trait-based approaches used in this study therefore improve our mechanistic understanding of community dynamics and are a step forward to higher predictability and generality in limnology.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:73169 |
| Date | 17 December 2020 |
| Creators | Wentzky, Valerie Carolin |
| Contributors | Borchardt, Dietrich, Rinke, Karsten, Schubert, Hendrik, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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